Sunrise effect on 40 kHz signal amplitude and its characteristic variation with respect to geomagnetic storms

The sunrise effect is a characteristic feature of very low frequency and low frequency radio waves propagated over a large distance. The 40 kHz signal level, transmitted from Miyakoji station (37.4° N, 140.9° E), Japan, and received at Tripura University (23° N, 91.4° E), is found to be attenuated during sunrise with an enhancement before the decrease in the signal level. On the basis of the nature of attenuation of the observed records from 2005 to 2006, those are classified into four different types, namely, type A (three step attenuation), type B (two step attenuation), type C (one step attenuation), and type D (no attenuation). During geomagnetically active days, 84% of type D cases and 31% of type C cases are observed, whereas only 0.9% of type A cases and 7% of type B cases are observed during geomagnetically active days. The fade amplitude of type C fade is also found to maintain a good negative correlation of 77.3% with the geomagnetic Ap indices over the period of 2 years. From the model calculation it is found that in the altitude range from 65 to 80 km, on average the electron density increases by a factor of 5.22 times during geomagnetically active days versus normal days.

A comparative study of the sunrise and sunset effects on the integrated field intensity of VLF atmospherics

A comparative study is made of the stepped structure of atmospherics during sunrise and sunset hours at three harmonically related frequencies in the VLF band, observed at the coast of the Bay of Bengal. The occurrences of the steps, their duration, time sequence and amplitudes are critically examined. Our analysis reveals that the start times of the third and second steps of the sunrise effect are always before ground sunrise, while the start of the main step is distributed on both sides of it. During sunset hours, the onset time of the main step is before the local ground sunset, but for the second and third steps, the onset times are after that. The results are interpreted by considering the multi-hop propagation as well as the distributed noise sources surrounding the observing site.